A conventional general combination of a screw and a screw driver bit, which is configured as shown in FIGS. 12 to 15, is known. That is, FIGS. 12 and 13 show a conventional screw having a cross groove, FIG. 14 shows a screw driver bit for this cross-grooved screw, and FIG. 15 shows the state of fitting of the above-described screw and the screw driver bit.
The conventional screw 10 shown in FIG. 12 is provided, on a screw head part 10a thereof, with a cross groove 12. This cross groove 12 is configured in such a manner that a specified tilted groove part 12a is formed in the direction from each end edge part so as to extend to the center part of a screw neck part 10b and, at the same time, in the bottom part thereof is formed a roughly conical bottom face 14 with a gentle inclination. Incidentally, in FIG. 12, reference numeral 13 each denotes a tapered side wall part formed between adjacent cross grooves 12. That is, the cutting-edge part of a screw driver bit, which will be described later, abuts and engages with this tapered side wall part 13. Furthermore, in the adjacent corner parts of each of the above-described tilted groove parts 12a are formed taper connection faces 17a, 17b, which extend from the position of the conical bottom face 14 to the opening edge part of the cross groove 12 of the screw head part 10a and also against these taper connection faces 17a, 17b abuts and engages a portion of the cutting-edge part of a screw driver bit, which will be described later.
On the other hand, a conventional screw driver bit 20 shown in FIG. 14 is configured in such a manner that the screw driver bit is provided with cutting-edge parts 22, each of which fits into the respective cross grooves 12 of the above-described screw 10, and there are formed extended cutting-edge parts 22a, each of which extends so as to match the shape of the tilted groove parts 12a formed in the direction from the end edge parts of the above-described cross groove 12 to the center part of the screw neck part 10b. Incidentally, in FIG. 14, reference numeral 23 denotes tapered side wall parts formed on both side faces of each of the above-described cutting-edge parts 22 and extended cutting-edge parts 22a. That is, these tapered side wall parts 23 abut and engage against the tapered side wall parts 13 formed in the cross groove 12 of the above-described screw 10.
According to a combination of the conventional screw 10 and screw driver bit 20 thus configured, as shown in FIG. 15, by causing the screw 10 and the screw driver bit 20 to fit into each other, as described above, each of the cutting-edge parts 22 of the screw driver bit 20 and each of the extended cutting-edge parts 22a fit into the tilted groove parts 12a of the cross groove 12 and the side wall parts 23 of the above-described cutting-edge parts 22 and extended cutting-edge parts 22a abut against the tapered side wall parts 13 in the cross groove 12 of the screw 10. Therefore, by rotating the screw driver bit 20, a prescribed torque can be transmitted to the screw 10. That is, it is possible to perform the attaching and detaching of the screw in a specified object to be mounted.
However, according to the combination of the conventional screw 10 and screw driver bit 20 configured as mentioned above, as shown in FIG. 15, the cross groove 12 of the screw head part 10a has a specified tilted groove part 12a which is formed in the direction from the end edge part of the cross groove to the center part of the screw neck part 10b and, on the other hand, the screw driver bit 20 which fits into the screw is configured in such a manner that the ridgeline part of the extended cutting-edge part 22a matches the shape of the above-described tilted groove part 12a and fits into the above-described cross groove 12 and, furthermore, this ridgeline part of the extended cutting-edge part 22a is formed in such a manner that the width increases gradually in the direction from the leading end thereof to the rear. In addition, also the tapered side wall part 23 formed in each cutting-edge part 22 of the screwdriver bit 20 abuts and engages with the tapered side wall part 13 formed in the cross groove 12 of the screw 10. Therefore, when the above-described screw driver bit 20 is rotated in a prescribed direction, the resulting contact between the above-described screw driver bit 20 and the cross groove 12 is such a state that can be said to be totally tapered contact, with the result that the leading end of the screw driver bit 20 tends to come out along the inclined plane of the tilted groove part 12a of the above-described cross groove 12 (indicated by an arrow in FIG. 15), leading to the occurrence of what is called a come-out phenomenon.
In particular, the shape of the cross groove 12 of the conventional screw 10 is such that, as shown in FIG. 13, in order to facilitate the fitting of the leading end of the screw driver bit 20, the groove width of the cross groove 12 is formed in a larger size than the width of the ridgeline part of the extended cutting-edge part 22a of the screw driver bit 20, whereas the areas of the tapered side wall part 13 and taper connection faces 17a, 17b formed in the boundary parts between adjacent cross grooves 12, 12 and in the corner parts thereof are relatively small. For this reason, when by the rotating operation of the above-described screw driver bit 20 large loads are applied to the above-described tapered side wall parts 13 and taper connection faces 17a, 17b and the screw tightening resistance is large, the above-described tapered side wall part 13 and taper connection faces 17a, 17b are gradually damaged as indicated by shaded portions 15 in FIG. 13. Therefore, if these damaged portions (shaded portions 15) are expanded, the occurrence of the come-out phenomenon of the above-described screw driver bit 20 becomes frequent, eventually making a screw tightening operation impossible.
Incidentally, the occurrence of this come-out phenomenon brings about the drawback that the cutting-edge part at the leading end of the screw driver bit is worn, for example, in the case of a tapping screw, because the design surface hardness of this screw is high. Also, in the case of a clutch-type automatic screw driver having a torque control function, there is a drawback that when the come-out phenomenon occurs during screw tightening, the operator cannot judge whether the screw has been securely attached with an appropriate torque.
In order to prevent the come-out phenomenon of the above-described screw driver bit 20 from these standpoints, in the rotation of the screw driver bit 20 it is necessary to apply a thrust which strongly pushes the screw driver bit against the screw groove 12a. However, although there is no problem when the object to which the screw is to be attached is a rigid body such as metal, in the case of a precision component etc. there is a drawback that these objects are damaged.
Also, the occurrence of the come-out phenomenon accelerates the wear of the leading end parts of the bit, i.e., the cutting-edge part 22 and extended cutting-edge part 22a, and the wear of these parts further promotes the occurrence of the above-described come-out phenomenon, with the result that there is a drawback that the damage to the screw groove also increases.
Furthermore, although it is possible to prevent the above-described come-out phenomenon by applying an excessive thrust to the above-described screw driver bit 20, on the other hand an accurate torque cannot be transmitted to the screw and the magnitude of torque applied to the screw driver bit 20 differs from operator to operator, with the result that there is a drawback that variations occur in the screw tightening torque.
On the other hand, when screw tightening is manually performed, it costs the operator great labor and fatigue to perform the operation of rotating the screw driver bit 20, with the screw driver bit being sufficiently pushed against the screw.
Also, according to the combination of the above-described conventional screw 10 and screw driver bit 20, when screw attaching is performed by use of a manual tool or motor-driven tool, in the fitting of the leading end parts of the bit into the screw groove it is difficult to perform the rotating operation of the screw while keeping the axis of the screw and the axis of the screw driver bit in a coaxial state. Therefore, when there is an inclination between the axis of the screw and the axis of the screw driver bit, there is a drawback that the above-described come-out phenomenon occurs frequently and that besides the damage to the screw groove also occurs frequently, thereby reducing the working efficiency of screw tightening operation and, at the same time, incurring waste of expenses caused by the consumption of damaged screws.
Moreover, also in the screw detaching work, the same come-out phenomenon and damage to the screw groove are apt to occur and, in this case, the detaching of the screw becomes impossible and hence it becomes unavoidable to break a portion of the object to which the screw is attached. In particular, in the case of occurrence of the clogging of the interior of the screw groove with dust and the like, the occurrence of the above-described situation becomes remarkable and, for example, there is a drawback that separation work for the recycling of waste materials involving the detaching of the screw becomes complicated.
Accordingly, the present inventors devoted themselves to conducting a series of research and trial manufacture and, as a result, found out that, in a screw which is formed so that a bit-fitting groove formed of a cross groove is provided in a screw head part, a specified tilted groove part is formed in the direction from the end edge part of the bit-fitting groove to the center part of a screw neck part, and a roughly conical bottom face is formed at a cross center part, by forming the end edge part of the above-described bit-fitting groove as a generally vertical end wall part of a specified depth, by forming a groove part with an inclination angle β of about 45° in the direction from the lower edge part of the end wall part to the conical bottom face at the center part of the screw neck part, and further by forming the conical bottom face with a gentle inclination angle α of approximately 28°, it is possible to securely prevent the come-out phenomenon of the screw driver bit during the fitting of the screw driver bit into the screw groove, to substantially reduce damage to the screw by increasing the strength of the screw, and besides to achieve well-balanced torque transmission to the screw.
It is therefore the object of the invention to provide a combination of a screw and a screw driver bit in which by improving the structure of a groove part in a cross groove of the screw, damage to the screw as in a conventional screw is prevented by effectively preventing the come-out phenomenon of the screw driver bit and in which even if the cross groove part of the screw is damaged, a proper and rapid screw tightening operation can always be achieved and the working efficiency can be remarkably increased, and a header punch for manufacturing the screw.